Quadruped Robot: Past, Present and Future

International Journal of Scientific Research and Review ISSN NO: 2279-543X

Quadruped Robot: Past, Present And Future

Nikunj S Yagnik#1, Dr. Mehul M Gor*2 #Automobile Engineering Department, ADIT, Gujarat Technological University *Mechanical Engineering Department, GCET, Gujarat Technological University

Abstract— As legged robots are most suitable to be operate in uneven terrain, it becomes more popular arena of research nowadays. Quadruped robot is superior among legged robots family in terms of optimization of speed and stability. Since long research on quadruped robot is carried out, however they are not commercially available. These shows there are lot of rooms of improvement and scope of research in quadruped robot. This paper discusses history of, early literature and contemporary development in the field quadruped robots. Challenges in the development and operation of quadruped robots are discussed. Few well known quadruped HyQ series, MIT Cheetah, ANYmal, BigDog, and StarlETH etc. are discussed in detail. Furthermore, various gait mechanisms, path movement methods, static and dynamic stability margins, power source, posture control, fault tolerant control and reconfiguration of these robots are discussed. Finally, the paper is concluded with the discussion on future development quadruped robots Keywords— Quadruped Robot, Legged Robot, Static Stability, Dynamic Stability.

I. INTRODUCTION

Legged robots have recognized to be a capable locomotion system, accomplished variety of tasks that conventional wheeled robot cannot do. Moreover, today’s reality is that it is the fast emerging field of study for researchers from a various field of engineering. Legs are indispensable part of humans or animals but development of legs for a robot is a tedious process[1]. In fact, how a baby learns to walk and the complete erudition curve involved was unnoticed. If we, the intellectual human being take years to learn to walk, envisage generating legs for a robot and teaching it how to walk. Even though, researchers are rigorously doing an extensive research on legged robot, but, they are still struggling to develop a legged robot which can duplicate human walk, or animals analogy for such matter. This paper is an overview of evolvement of specific quadruped (four legged) robot over period of time with future scope[2,3]. Developing legged robot is very changing composite task, but there are many advantages of legs over wheels. If we see all our living organism, they all carries legs rather than other mechanism, so as, they can adapt to any environment and different terrain. Legged robots have the biggest problem of stability and speed. Wheeled robots have biggest problem of locomotion on un-even terrain. Legged robots can steer on any kind of surfaces which is unreachable for robots with wheels. It is known to us that wheels are designed to work on organized surfaces like roads, smooth surfaces, prepared tracks/rails, etc., whereas legged robot has ability to move on variety of terrain. Legged robots can step up/down and jump or clear over obstacles whereas wheels need to change the part or somehow travel over it with ups and down. Wheels can move over a continuous path to be travel whereas legs can step over remote paths and move on. For example, if any particular city is hurt by an earthquake, which makes road surface discontinuous. In such incident, legged robots come into picture, which can make over uneven terrain. In many cases wheels with track can be also one of the solution. Legged robots can avoid undesirable footholds which cannot be avoided in a wheeled robot. Human and animal locomotion can be explore while development of the Legged robot. If we consider the steep, sandy, rocky, and undesired terrain, it is obvious that our brilliant invention of wheels turn out absolutely useless. Legged robot can be classify by number of legs been added to robot itself like One legged robot, Two legged robot (Biped), Three legged robot (Triped), Four Legged robot(Quadruped), and Six Legged Robot (Hexapod). One legged robot, name itself implies carry one leg which having a discrete ground contact. One legged robots are generally designed to be hopping robots. Such robots are dynamically stable and balancing can be done by changing its center of gravity and applying corrective forces to avoid falling when troubled. The typical benefit of such one legged robot is that every time they jumping up and move forward which makes them to move on any surfaces and overcome obstacles. Many times they can jump up larger, even more than ideal step. In one legged robot, controllable leg is only one so as developer can avoid leg coordination. In case of energy efficiency one legged robot is more efficient than robot with more number of legs. The major disadvantage is it’s complex design and control. Robots with two legs known as Biped robots, looks as human and continuously developed by various companies generally known as humanoids. These robots are dynamically stable and needs complex control algorithms to balance them when standing. Many such robots already been developed for example Honda’s “ASIMO” and Sony’s “QRIO” which can walk, run, stand and even dance. Three legged robots carries three legs which makes them statically stable because there are three contact points to ground. For static stability minimum requirement of three contact points to ground. For example the robot developed by RoMela knows as STriDer (Self-excited Tripedal Dynamic Experimental Robot) had three legs.

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One can easily walk with four legs which is more common in most of all animals that is why there is a good reason to replicate this in robots too. As like three legged robot, four legged robots are also statically stable (four contact point to ground)and the possible walking pattern of a four legged robot can be designed by either one leg at a time or alternative pairs. More number of legs provides greater stability. Hexapods (robots with 6 legs) possess greater static stability while moving and standing too. These robots are also biologically inspired as there are many insects with 6 legs. Most 6 legged robots mimic spiders, either wave gait or follow tripod gait.

II. INCEPTION OF QUADRUPED ROBOT/MACHINE There is a long history associated with the evolution of walking machines/robot [3–6]. From the traditional time, world was inquisitive about development of artifacts that resembles to the animals and human. automatically steam-powered walking machines and mechanisms within the disguise of animals and human look may be derived back throughout the time of Greek civilization. abstract style and sketches of artiﬁcial systems and mechanisms capable of human locomotion appeared in Europe throughout the time of renaissance. The interest to understand the technicalities of legged locomotion discovered in nature and also the efforts to duplicate them to mobilize the artifacts by leg mechanisms are mentioned within the mythology and ancient scripts from the traditional Greek, Indian, Egyptian, and Chinese civilization[2].Although no technical details of the style and development of those ancient walking devices and also the associated mechanisms area unit accessible, the imagination and ideas propounded by them area unit quite fascinating and stimulating. The ﬁrst quadruped machine, proposed by L. A. Rygg in 1893, named “The Mechanical Horse” [3, 10], Fig 1. The Mechanical Horse was proprietary on fourteen Feb 1893. The mechanism shows the stirrups were used as pedals in order that the rider might power the mechanisms. The movement from the pedals was transmitted to the legs through gears that might result the stepping motions. However, there's no proof to prove that he really designed this machine.

Figure 1 First Quadruped Machine : Mechanical Horse[10]

Bechtolsheim Baron patented a Quadruped machine in 1913. The proposed design is given below Figure 2. However, no evidence is available which shows such machine actually built.

Figure 2 Baron Quadruped Machine [10]

From the preceding dialogue it's clean that numerous novel ideas related to artificial taking walks mechanisms emerged from the conventional instances until the nineteenth century. even though the event of mechanisms dominated in maximum of the cases, other aspects of strolling machines like effort, sensing, and adjust were given little or no interest from the developers due to the technological bottleneck prevailing at the up to date quantity. however these inventions were nice inspirations for

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subsequent generation artificial intelligence engineers for the planning and development of extra advanced and extra delicate autonomous walking machines to perform the supposed tasks. It was marked by an intensive development of varied walking machines, and therefore the history of walking machines when the Second war is very quick and biological process and might be delineate among subsequent section.

III. INITIAL DEVELOPMENT OF MODERN SCIENTIFIC WALKING ROBOTS After second world war, during 1950’s, a number of analysis teams started and develop walking machines in an exceedingly organized manner. Regarding the last decade later, various types of legged machines began to be designed and developed by totally different teams in laboratories. In 1960, Shigley, rigorously study linkage mechanism of walking locomotion. He proposed several mechanism name as Four bar chain, Cam linkages, Pantograph mechanism which can be useful for legged robots mobility. He developed a vehicle with four rectangular frame in which frames worked as leg. To ensure static stability distance between legs kept short as possible and two legs moves in pair for locomotion. The motion of legs was regulated by set of double rocker linkages and needs non circular gears for uniform velocity of foot motion. Ralph Mosher, Engineer of General electricals’ General Engineering laboratory ab initio started developing a quadruped vehicle/truck well-known as “General Electric quadruped” in 1960,Fig 3. He has completed the project by 1968,the quadruped having 3 degree of freedom per leg –two in the hip and one in the knee which is being actuated through crank by a hydraulic cylinder. This vehicle was propelled by 68 KW IC engine having length 3.0m,height 3.3m and weight 1400kg[10]. The skilled operator is required to control the machine properly. The pedals were hydraulically connected legs which were controlled by operator with the help of joysticks. As one leg having 3 DOF means total 12 DOF should be controlled by the operator makes this task very challenging and they got tried after sometimes. Even though, such initial developed quadruped invention was very important for contemporary walking robots as a result of it incontestible a walking of a machine which may surpass obstacles with smart quality in several terrains as well as provided versatile gait. However, there is need of computer control system for the actuation of legs as we as for the coordination.

Figure 3 GE Quadruped The most important development in terms of walking quadruped robot came in the same time period,1966-68, by Frank and Mcghee, at University of Southern California who created “Phony Phony” four legged robot, Fig 4. It was the first legged robot with computer controlled and electrical actuations with two DOF per leg. The leg mechanism has 2 links and 2 joints. each joints are driven by electrical motors with worm gears speed reduction method and externally power through cable. This quadruped had two gait configuration name as : Walk and trot. However, it having one drawback in terms of mobility only in straight line. It cannot take turn.

- Figure 4 Phony Phony-First Computer controlled Quadruped[10]

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IV. CURRENT DEVELOPMENT OF QUADRUPED ROBOTS The recent development of quadruped robot by like TITAN series, Big-Dog, HyQ series, MIT Cheetah, ANYmal, and StarlETH is incorporated in this section. The fig. 5 shows quadruped robot developed by Hirose and his Colleagues.

Figure 5 TITAN III,TITAN IV, (1981,1985) [20]

Figure 6 TITAN XIII(2005) TITAN III has four legs, each 1.2m long and weights 80 kg. It has total 12 DOF, 3 DOF per leg with generalized crawl gait that allow it to move in omni-direction. When robot step into uneven surfaces it uses free gait, that may be a non-periodic gait to look for safe footholds, avoiding obstacles and maintaining static stability. While returning to flat ground, it is controlled by crab-walk gait efficiently. TITAN IV has same configuration but weights 160 kg. It used more powerful DC motor compare to TITAN III.[17] This robot stands on two legs at all the times, i.e. trot gait. A trot gait is dynamic walk wherever the zero moment point ought to be controlled to be within the support pattern. Later, TITAN VIII and XIII was developed by the same team in which leg mechanism consists of a planar a pair of degrees of freedom mechanism and a mechanism that rotates this mechanism [20]. So as, it had 3 DOF per leg and wires with pulleys are used for power transmission within leg. The main properties of recent TITAN series robots is given below in tabular form. TITAN – XIII TITAN - VIII Size 420*420*300 400*600*250 Weight (without 4.85 kg 22 kg Battery) Weight (with Battery) 5.2 kg 27 kg Payload 1 kg 5 kg Max. Walking 1 m/s 0.4 m/s Velocity Battery Run Time 0.5 h 0.1 h Total output of Motor 822 W 1080 W Power to Weight Ratio 170 W/kg 50 W/kg Table 1. TITAN XIII/VIII Comparison [17,20] The next quadruped robot name as Big DOG, developed by Boston Dynamics in 2005 by Foster-Miller. It is an energy autonomous hydraulic quadruped robot with 1 m tall,1 m long and weight 90kg as shown in fig.7. Each leg has one passive linear pneumatic compliance within the lower leg, and 3 active joints for knee,hip pitch and roll. Big-Dog can walk up and down 350 inclines, can trot at speeds of up to 10 kmh and can carry over 150kg of payload. It is dynamically balance robot. In 2012, legged squad support system variant of Big-Dog developed which having capabilities during a hike over tough terrain. BigDog is powered by 2-stroke,one cylinder,15BHP go kart engine operating at over 9000rpm [15].

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Figure 7 Big Dog (2005) [15] BigDog’s regulating system keeps it balanced, manages locomotion on a large form of surfaces/land, and will navigation. Sensors for locomotion embrace joint position, joint force, ground contact, ground load, a gyro, measuring device and a stereo vision system. Different sensors target the inner state of BigDog, observance the hydraulic pressure, oil temperature, engine functions, battery charge etc. The another quadruped robot is MIT’s Cheetah 3 robot which is “ a Ferrari in the robotics world”, Fig 8. The robot, known as cheetah will run on batteries at speed of quite thirty mph, jump concerning sixteen inches high, land safely and continue pace for a minimum of quarter-hour. Its weight is 70 pound. It will climb stairs and step over obstacles while not the assistance of cameras or visual sensors. The cheetah had powerful, lightweight motors, electronics that control power for its 12 motors (3 motors/DOF per leg)and an algorithm that determines a sum of force a leg should exert during the schism second that it spends on the ground while run to maintain balance and forward momentum. An onboard computer organizes information from various sensors and sends instruction to each motor. Sensors within the robot detect the angle of the leg and such information is sent to an onboard computer that also categorizes data from Inertial Measurement Unit, which is used in Pilotless aircrafts/Drones and Ballistic missiles.

Figure 8 Cheetah 3 (2014) The next quadruped robot name as ANYmal is designed for autonomous operation in challenging environments, developed by ETH Zurich; Robotics System Lab as shown in Fig 9. It is driven by special compliant and precisely torque controllable actuators, the system is capable of dynamic running and high-mobile climbing. It used optical maser sensors and cameras, the robot can observe its situation to continuously create maps and accurately localize. With the help of such information, it can autonomously plan its navigation path and carefully select footholds while walking. It can be used in application like industrial inspection of oil and gas sites. ANYmal carries batteries for over 2h autonomy and totally different sensory instrumentation like optical and thermal cameras, microphones, gas-detection sensors and active lighting. With this payload, the machine weighs but 30kg and may therefore be simply transported and deployed by one operator and even carry load up-to 10kg too. It had robust locomotion with different gaits like walking and trotting. It had joint mobility for knee and hip both 3600.

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Figure 9 ANYmal (2014)

The another quadruped robot developed by ETH Zurich, known as, StarlETH at laboratory of University of Michigan, shown in given below figure 10. It was developed for fast, efficient, and versatile locomotion on four legs. It was developed with high compliant elastic legs. It had different gaits, such as bounding, trotting, or running. Robot’s 4 identical legs that are arranged in an X conﬁguration. The linear dimensions of about 0.6m and a total weight of 23kg, seems like a medium-sized dog.

Figure 10 StarlETH (2012)

Each leg has a total of 3 degrees of freedom, one each for hip abduction/adduction, hip ﬂexion/extension, as well as knee ﬂexion/extension. It can carry load up-to 20kg. StarlETH had very high torque capability, its compliant protection against impact collisions, and full torque controllability with control algorithm and CAN system. The HyQ series quadruped robot development started at advanced robotic department, Italian Institute of Technology in 2007. The HyQ is a hydraulically actuated quadruped robot, which can run, jump, and moreover as travel within the uneven terrains. The significance to developed this robot was to have a versatile quadruped robot that is able to perform highly dynamic motions with precise navigation over rough terrain. Since 2007, extensive research and testing had been completed which bring last version of HyQ 2Max can able to walk, run and jump. Rigid body dynamics, Motion planning, low/high level control, etc. are part of ongoing research. The HyQ robot weighs 90 kg with the hydraulic power supply on board and 70 kg with the external hydraulic power supply. It is 1 m in length, 50 cm in width and 98 cm in height. It is made of stainless steel and is covered with 3 millimeters thick sheet of aluminum alloy[4]. HyQ has four lightweight legs that are actively compliant. It has a total of 12 joints, and each leg has three degrees of freedom to provide high flexibility for each move. The hydraulic cylinders operate eight leg joints, while the other four joints operated by BLDC motors.[19]

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Figure 11 HyQ Series 1.0/2.0 (2007-2015)[04]

Each joint is integrated with loading cells and encoders to control the torque and leg position. It also uses actuators to absorb shocks and vibrations while carrying out heavy tasks. Due to hydraulic actuation, it can provide increased velocity, power, and torque production.

V. CHALLENGES IN THE DEVELOPMENT AND WORKING OF WALKING OF QUADRUPED ROBOT The main challenges in the development of an autonomous walking robot[ 9, 19] are: 1) Lightweight and robust material to build robot structure and mechanism 2) Energy Efficient and high performance actuators with higher power to weight ratio. 3) Fast, high computing power 4) On board power source with longer time duration. 5) Advanced, economical, Reliable sensors.[17] Various other aspects of recent walking robotics research is the development of biologically inspired design like MIT’s Cheetah, which includes the idea of transferring to the robotic systems the biological principles of nature[26–34].Initially, While designing a walking robot one may be tempted to imitate a legged animal. It is not possible to completely imitate a living system, however, because the principles of energy supply, performance, sensing, control and intelligence of a biological system are completely different from artificial energy supply, actuation, sensing, control, and intelligence. Since the concepts of the biological world are difficult to transfer to walking machines, mainly due to the mechatronic bottleneck, many laboratories and their researchers have recently been involved in the design and development of artificial locomotion systems with the appropriate modification of the principles perceived in the biological world to accommodate the existing mechatronic technology. Researchers in robotics should pay attention to many advanced technologies, such as nanotechnology, micro- electromechanical systems, single- chip computers and smart materials, which can help man to develop a walking robot similar to living legged systems. Furthermore, the reconfigurable Quadruped robots which has both wheels as well as legs in built. Such robots has been developed recently name as Chariot III [13], Whegs [27], Impass [28] and PEOPLER-II [25]. However, there is need for improved novel efﬁcient mechanical and physiological designs which can be easily switch over two mechanism for wheel to leg and leg to wheel. The ability to automatically reconfigure and redesign control systems is enhanced by several technological developments such as (i) measurements from high-quality sensors, (ii) more information about the diagnostic and redesign algorithms, and (iii) the continuing increase in computing power and speed. Fault-tolerant control involves both automatic system identification and control system redesign. Another recent area of research is to study failure of elements of robot and such fault has to be resolve by self-reconfiguration or system should be fault tolerant [25,35]. For multi- legged walking robots, the possible failure situations can be classified as:( i) failure in the kinematic part of the leg and( ii) failure to communicate between the controller and the leg. The failure of the cinematic part of the leg can usually be one of the next two types. The first is a free swing failure, which loses the actuator torque of a leg joint.. The second is a locked joint failure for which a joint cannot move and is locked, but the ability of the leg to support the body is maintained., i.e., The failed leg still partially contributes to the locomotion of the robot. Locked joint failure is more common because of the friction in bearings, clutch slipping, brakes applied due to problem with fault detection software etc. The failures can be taken care either by the fault tolerant control or fault accommodation through reconfiguration of walking robot. The first strategy ‘fault tolerance control’ concerns with changing the control laws and the associated control problem to maintain the desired output. Fault accommodation through reconfiguration is accomplished by adopting control laws, alternative standby devices called hardware redundancies instead of defective components are deployed. Some strategies of fault tolerant control and reconfigurations of legged robots are reported in literature [25,34], however, there is a need of developing such strategies for hybrid robot also.

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VI. CONCLUSION In this paper, several developed walking quadruped robots characteristics with specification are discussed Also the requirements and limitations made clear too. Furthermore, various other aspects like gait mechanism, stability along with reconfiguration and fault tolerant system been also incorporated briefly. Moreover, development of next generation walking robots is investigated by adding novel feature to it, like automatic reconfiguration and fault tolerant mechanism.

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